I can't get any music that I didn't buy from itunes to sync to my ipod. Any suggestions?

I just bought some music from Amazon mp3, and also ripped some from CDs. I can't get any of this new stuff to sync to my ipod. It's in my itunes library, but when I sync my ipod, it appears greyed out with the little dotted circle. Any ideas as to why this music won't actually download to my ipod?

Thanks!